Stabilized monomer composition

ABSTRACT

A monomer composition is described, containing A) vinyl-containing monomers in which the vinyl bears a hetero-atom selected from the group consisting of the halogens, nitrogen, oxygen, sulfur or silicon, and 
     B) at least one N-oxyl compound of a secondary amine which does not bear hydrogen on the α-carbons, in an amount effective as stabilizer against premature polymerization.

The present invention relates to monomer compositions containing

A) vinyl-containing monomers in which the vinyl bears a hetero-atomselected from the group consisting of the halogens, nitrogen, oxygen,sulfur or silicon, and

B) at least one N-oxyl compound of a secondary amine which does not bearhydrogen on the α-carbons, in an amount effective as stabilizer againstpremature polymerization.

The invention further relates to processes for inhibiting the prematurepolymerization of vinyl-containing monomers and to the use of N-oxylcompounds of secondary amines which do not bear hydrogen on theα-carbons as stabilizers against premature polymerization.

To prevent premature polymerization, it is necessary to add stabilizersto the monomers. Those which have proved particularly suitable forpreventing free-radical polymerizations are sterically hindered amines,such as 2,2,6,6-tetraalkylpiperidine, and their derivatives, includingalso the N-oxyl derivatives.

U.S. Pat. No. 5,254,760 describes the stabilization of vinylaromaticcompounds such as styrene during distillation and purification by acombination of at least one nitroxyl compound and at least one aromaticnitro compound, in which there is the risk that traces of the nitroxylcompounds pass into the purified monomer. Even traces of nitroxylcompounds interfere with the subsequent polymerization, however; theycause delayed polymerization and uncontrolled chain breaks, leading topolymers which are insufficiently reproducible and are of short chainlength. These disadvantageous effects are described by Mardare et al. inPolym. Prep. (Am. Chem. Soc., Div. Polym. Sci.) 35 (1), 778 (1994).

To stabilize heterosubstituted vinyl compounds such as N-vinylformamideor N-vinylpyrrolidone, derivatives of phenylenediamine (U.S. Pat. No.5,396,005), fullerenes (DE-A 44 14 773) or 2,6-di-tert-butyl-p-cresole(DE-A 43 28 950), for example, have previously been used in distillationand purification, but the efficiency of these stabilizers is consideredto be inadequate.

It is an object of the present invention, therefore, to find monomercompositions which contain heterosubstituted vinyl compounds andsuitable stabilizers, which display improved stabilization againstpremature polymerization and which have virtually no disadvantageouseffects on the following targeted polymerization of the monomers.

We have found that this object is achieved by the monomer compositionsmentioned at the outset.

The heterosubstituted vinyl monomers preferably bear, as hetero-atom onthe vinyl group, halogens, oxygen, nitrogen or sulfur.

Hetero-substituted vinyl monomers which are suitable are, for example,vinyl halides, such as vinyl chloride, vinyl esters of carboxylic acids,such as vinyl acetate, vinyl propionate or vinyl butyrate, vinyl ethers,such as methyl vinyl ether, ethyl vinyl ether or butyl vinyl ether,vinyl thioethers, vinyl carbazoles, vinyl pyrrolidones, vinylphthalimides, vinyl isocyanates, vinylcaprolactams, vinylimidazoles,vinylformamide, vinylsulfonic acid and vinylsilanes, such asvinyltriacetoxysilane, vinyltrichlorosilane or vinyltrimethoxysilane.

Preferred monomer compositions contain

A) monomers of the general formula I

    CH.sub.2 ═CH--X--R.sup.1                               I,

where

X is oxygen or --NR² --,

R¹ is ##STR1## or --R³ , R² is hydrogen, C₁ -C₄ -alkyl or, together withR³, is a saturated or unsaturated C₃ -, C₄ - or C₅ -alkylene bridge inwhich up to two CH₂ groups can be replaced by NH, N(C₁ -C₄ -alkyl), N(C₆-C₁₀ -aryl) or oxygen and up to two CH groups can be replaced by N and

R³ is hydrogen, C₁ -C₄ -alkyl or a radical which, together with R², is asaturated or unsaturated C₃ -, C₄ - or C₅ -alkylene bridge in which upto two CH₂ groups can be replaced by NH, N(C₁ -C₄ -alkyl), N(C₆ -C₁₀-aryl) or oxygen and up to two CH groups can be replaced by N, and

B) at least one N-oxyl compound of a secondary amine which does not bearhydrogen on the α-carbons in an amount effective as stabilizer againstpremature polymerization.

The monomers (A) of the general formula I which are present in themixtures of the invention can contain oxygen as the variable X. Monomersof this type which are particularly suitable as a constituent of thenovel monomer compositions are vinyl ethers in which R¹ is C₁ -C₄-alkyl, i.e. methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl or tert-butyl.

If the variable X is --NR² --, R¹ is preferably--CO--R³.

Radicals R³ which are suitable are, in addition to hydrogen and said C₁-C₄ -alkyl groups, those radicals which form, together with --NR² --, asaturated or unsaturated 5- to 7-membered ring, for example: ##STR2##including particularly N-pyrrolidinonyl and N-caprolactamyl.

Preferred monomers in the compositions of the invention areN-vinylformamide, N-vinyl-2-pyrrolidone, N-vinyl-ε-caprolactam and theabovementioned C₁ -C₄ -alkyl vinyl ethers.

Particular preference is given among these monomers to N-vinylformamide.

As stabilizers (B), the novel monomer compositions contain at least oneN-oxyl compound of a secondary amine which does not bear hydrogen on theα-carbons. These compounds can be present as the free form or as theirsalts.

Suitable N-oxyls of amines are eg. the following structures ##STR3##where R is identical or different alkyl, cycloalkyl, aralkyl or aryl,which can also be joined in pairs to form a ring system, and Y is agroup which is necessary to complete a 5- or 6-membered ring. R is, forexample, C₁ -C₂₀ -alkyl, in particular C₁ -C₈ -alkyl, C₅ - or C₆-cycloalkyl, benzyl or phenyl. Y is, for example, an alkylene group--(CH₂)₂ -- or --(CH₂)₃ --.

Furthermore, N-oxyl compounds are suitable, such as those having thefollowing structures ##STR4## where the aromatic rings can each furtherbear from 1 to 3 inert substituents, such as C₁ -C₄ -alkyl, C₁ -C₄-alkoxy or cyano.

Preferably, sterically hindered cyclic amine derivatives are used, eg.of piperidine or pyrrolidine compounds which can contain a furtherheteroatom in the ring such as nitrogen, oxygen or sulfur, where thisheteroatom is not adjacent to the hindered amine nitrogen. The sterichindrance is due to substituents in the two adjacent positions to theamine nitrogen, suitable substituents being hydrocarbon radicals whichreplace all 4 hydrogens of the α--CH₂ groups. Specific examples of suchsubstituents are phenyl, C₃ -C₆ -cycloalkyl, benzyl and, in particular,C₁ -C₆ -alkyl, where the alkyl radicals bound to the same α-carbon canalso be joined together to form a 5- or 6-membered ring. Particularpreference is given to the radicals listed in detail under R⁴ and R⁵.Preferably, the N-oxyls of sterically hindered amines used arederivatives of 2,2,6,6-tetraalkylpiperidine.

Preferred N-oxyl compounds in the monomer compositions of the inventionare those of the general formula II ##STR5## where R⁴, R⁵ are C₁ -C₄-alkyl, phenyl or, together with the carbon to which they are bound, area 5- or 6-membered saturated hydrocarbon ring,

R⁶ is hydrogen, hydroxyl, amino or an m-valent organic radical bound byoxygen or nitrogen or, together with R⁷, is an oxygen or a ringstructure defined under R⁷ ,

R⁷ is hydrogen, C₁ -C₁₂ -alkyl or, together with R⁶, is oxygen ortogether with R⁶ and the carbon to which they are bound, is thefollowing ring structures ##STR6## where, for the cases when R⁶ jointlyforms a radical with R⁷, m=1,

R⁸ is hydrogen, C₁ -C₁₂ -alkyl or --(CH₂),--COOR⁹,

R⁹ is identical or different C₁ -C₁₈ -alkyl,

k is 0 or 1

z, p are from 1 to 12 and

m is from 1 to 100.

R⁴ and R⁵ can be, for example, the C₁ -C₄ -alkyl groups mentioned forR¹, or they can together form tetramethylene or pentamethylene.Preferably, R⁴ and R⁵ are methyl.

Suitable examples of R⁷ are hydrogen, the abovementioned C₁ -C₄ -alkylgroups, and also pentyl, sec-pentyl, tert-pentyl, neopentyl, hexyl,2-methylpentyl, heptyl, 2-methylhexyl, octyl, isooctyl, 2-ethylhexyl,nonyl, 2-methylnonyl, isononyl, 2-methyloctyl, decyl, isodecyl,2-methylnonyl, undecyl, isoundecyl, dodecyl and isododecyl, (the namesisooctyl, isononyl and isodecyl are trivial names and are derived fromthe carbonyl compounds obtained by oxosynthesis; c.f. in this contextUllmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A1,pages 290-293, and Vol. A10, pages 284 and 285).

p is preferably 6-12, particularly preferably 9.

z is preferably 1-4, particularly preferably 2.

Examples of R⁸ which are suitable are, in addition to hydrogen, theabovementioned C₁ -C₁₂ -alkyl groups. Preferably, R⁸ is hydrogen, C₁ -C₄-alkyl or (CH₂)_(z) --COO(C₁ -C₆ -alkyl), particularly preferably --CH₂-CH₂ --COO(CH₂)₁,--CH₃ and --CH₂ --CH₂ --COO(CH₂)₁₃ --CH₃.

R⁹ can be, for example, one of the abovementioned C₁ -C₁₂ -alkyl groupsor tridecyl, isotridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecylor octadecyl. Preference is given to dodecyl and hexadecyl.

Examples of preferred radicals R⁶ are the following m-valent radicals##STR7## where R¹⁰ is C₁ -C₁₂ -alkyl or --(CH₂)_(z) --COOR⁹

R¹¹ is hydrogen or C₁ -C₁₈ -alkyl,

R¹² is C₁ -C₁₈ -alkyl, vinyl or isopropenyl,

R¹³ is C₈ -C₂₂ -alkyl,

R¹⁴ is hydrogen or an organic radical such as is customarily formed inthe free-radical polymerization of the starting monomers,

k is 0 or 1,

x is from 1 to 12 and

n is an even number m.

If R⁶ is one of these radicals, R⁷ is preferably hydrogen. the variablem in this case can be from 1 to 100, preferably 1,2,3,4 or a number from10 to 50, mixtures generally being used, particularly in the case of theoligomeric or polymeric radicals R⁶.

As R¹⁰, the same radicals are suitable as are mentioned for R⁸.Preferably, R¹⁰ is C₁ -C₄ -alkyl.

As R¹¹, in addition to hydrogen, the same radicals are suitable as havebeen mentioned for R⁹. Preferably, R¹¹ is hydrogen.

As R¹², vinyl, isopropenyl or C₁₅ -C₁₇ -alkyl are particularly suitable.

Examples of R¹³ which are suitable are the abovementioned C₈ -C₁₈ -alkylradicals and nonadecyl, eicosyl, uneicosyl and doeicosyl, mixtures ofvarious radicals R¹³, which differ in the length of the carbon chainbeing preferred.

The radicals R¹⁴ are hydrogen or organic radicals such as are formed inthe free-radical polymerization of the starting monomers, in this casefrom an ethylene derivative and a maleimide derivative, i.e. a radical,for example, which is formed from the polymerization initiator or from afree radical which occurs as an intermediate, or another radical of thistype as is customary to those skilled in the art.

Preferred nitroxyl compounds as component B) of the novel monomercompositions are also the following:

1-oxyl-2,2,6,6-tetramethylpiperidine,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-one,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl acetate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 2-ethylhexanoate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate,

1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl (4-tert-butyl)benzoate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) succinate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) adipate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) sebacate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) n-butylmalonate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) phthalate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) isophthalate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) terephthalate,

bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) hexahydroterephthalate,

N,N'-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipinamide,

N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)caprolactam,

N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)dodecylsuccinimide,

2,4,6-tris-[N-butyl-N-(1-oxyl-2,2,6,6,-tetramethylpiperidin-4-yl]-s-triazine,

4,4'-ethylene-bis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one) and

tris-(2,2,6,6-tetramethyl-1-oxylpiperidin-4-yl) phosphite.

The nitroxyl compounds described may be prepared from the correspondingpiperidine compounds by oxidation, e.g. with hydrogen peroxide. Detailsof this oxidation are mentioned, for example, in the earlier GermanPatent Application 195 101 84.7. The secondary amines which do not bearhydrogen on the α-carbons, such as piperidine compounds, and theirpreparation, are generally known.

Since the oxidation reactions do not always proceed completely, thepiperidine compounds acting as starting compounds and partially oxidizedintermediates can also be present in the monomer compositions of theinvention.

Particularly suitable monomer compositions are those which, apart fromthe nitroxyl compounds mentioned, additionally contain one or morearomatic nitroso or nitro compounds for stabilization.

Examples of aromatic nitro compounds which can be used are1,3-dinitrobenzene, 1,4-dinitrobenzene, 2,6-dinitro-4-methylphenol,2-nitro-4-methylphenol, 2,4,6-trinitrophenol, 2,4-dinitro-1-naphthol,2,4-dinitro-6-methylphenol, 2,4-dinitrochloroberzene, 2,4-dinitrophenol,2,4-dinitro-6-sec-butylphenol, 4-cyano-2-nitrophenol,3-iodo-4-cyano-5-nitrophenol, particularly preferably2,6-dinitro-4-methylphenol, 2-nitro-4-methylphenol,2,4-dinitro-6-sec-butylphenol or. 2,4-dinitro-6-methylphenol.

Suitable aromatic nitroso compounds are, for example, p-nitrosophenol,p-nitroso-o-cresole and p-nitroso-N,N'-diethylaniline.

In addition, substituted phenols can also be added to the monomercompositions as costabilizers, such as for example the following:

4-tert-butylcatechol, methoxyhydroquinone,2,6-di-tert-butyl-4-methylphenol, n-octadecylβ-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,1,1,3-tris-(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,3,5-tris-(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate,

1,3,5-tris-[β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxyethyl]isocyanurate, 1,3,5-tris-(2,6-dimethyl-3-hydroxy-4-tertbutylbenzyl)isocyanurate and pentaerythritoltetrakis-[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

Furthermore, the novel monomer compositions may also contain one or morecostabilizers selected from the group consisting of phenothiazines,quinones, hydroxylamines or phenylenediamines.

To stabilize the monomer compositions of the invention, thesecompositions generally contain from 0.0002 to 5, preferably from 0.0005to 0.5% by weight of the nitroxyl compounds, based on the total amountof the monomer composition.

The stabilizers display their stabilizing action in a broad temperaturerange, being active at any conventional storage temperature from -50 to+50° C. and likewise at elevated temperatures, as employed, for example,in the distillation of the monomers. The pressure range of thestabilizing process is not critical either. The stabilizers act atatmospheric pressure and also at reduced pressure, as is sometimesemployed in distillation processes.

The process of the invention for inhibiting premature polymerization ofmonomers is used during preparation, distillation or purification of themonomers and also in their storage and transport.

EXAMPLE

Storage of N-vinylformamide in the presence ofN,N'-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N'-bis-formyl-1,6-diaminohexane

A monomer composition of vinylformamide and 0.05% by weight ofN,N'-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,N'-bis-formyl-1,6-diaminohexane,based on the total amount of the composition, was stored at 40° C. Afterthe times specified in the table, a sample was withdrawn and the contentof vinylformamide was determined by iodometric titration. The iodinevalues determined are listed in the table as a measure of the respectivecontent of vinylformamide.

    ______________________________________                                        Storage time                                                                              Iodine value without                                                                       Iodine value with                                    [days]      stabilizer   stabilizer                                           ______________________________________                                         0          98.8         98.2                                                 20          89.9         92.8                                                 41          80.5         87.5                                                 62          71.5         81.2                                                 83          62.5         75.7                                                 ______________________________________                                    

What is claimed is:
 1. A monomer composition, comprising:a)vinyl-containing monomers in which the vinyl bears a heteroatom selectedfrom the group consisting of nitrogen, oxygen, sulfur and silicon; andb) at least one N-oxyl compound of a secondary amine which does not bearhydrogen on the α-carbons, in an amount effective as a stabilizeragainst premature polymerization.
 2. The monomer composition of claim 1,wherein said vinyl-containing monomers have the formula (I):

    CH.sub.2 ═CH--X--R.sup.1                               (I)

wherein X is oxygen or --NR² --; R¹ is ##STR8## or --R³ ; R² ishydrogen, C₁ -C₄ -alkyl or, together with R³, is a saturated orunsaturated C₃ -C₄ - or C₅ -alkylene bridge in which up to two CH₂groups can be replaced by NH, N(C₁ -₄ -alkyl), N(C₆ -C₁₀ -aryl) oroxygen and up to two CH groups can be replaced by N; and R³ is hydrogen,C₁ -₄ -alkyl or a radical which, together with R², is a saturated orunsaturated C₃ -C₄ - or C₅ -alkylene bridge in which up to two CH₂groups can be replaced by NH, N(C₁ -C₄ -alkyl), N(C₆ -C₁₀ -aryl) oroxygen and up to two CH groups can be replaced by N.
 3. The monomercomposition of claim 1, wherein said vinyl-containing monomers areselected from the group consisting of N-vinylformamide,N-vinylpyrrolidone, N-vinylcaprolactam, and C₁ -C₄ -alkyl vinyl ether.4. The monomer composition of claim 1, wherein the vinyl-containingmonomer is N-vinylformamide.
 5. The monomer composition of claim 1,wherein the vinyl-containing monomer is vinyl chloride.
 6. The monomercomposition of claim 1, wherein the at least one N-oxyl compound has theformula (II): ##STR9## wherein R⁴, each R⁵ are each C₁ -C₄ -alkyl,phenyl or, together with the carbon to which they are bound, are a 5- or6-membered saturated hydrocarbon ring;R⁶ is hydrogen, hydroxyl, amino oran m-valent organic radical bound by oxygen or nitrogen or, togetherwith R⁷ is oxygen or a ring structure as defined for R⁷ ; R⁷ ishydrogen, C₁ -C₁₂ -alkyl or, together with R⁶, is oxygen or, togetherwith R⁶ and the carbon to which they are bound, is the following ringstructures ##STR10## wherein when R⁶ jointly forms a radical with R⁷,m=1; R⁸ is hydrogen, C₁ -C₁₂ -alkyl or --(CH₂)_(z) --COOR⁹ ; R⁹ isidentical or different C₁ -C₁₈ -alkyl; k is 0 or 1; z, p are from 1 to12; and m is from 1 to
 100. 7. The monomer composition of claim 6, whereR⁶ in formula (II) is a radical selected from the group consisting of:##STR11## wherein R¹⁰ is C₁ -C₁₂ -alkyl or (CH₂)₂ --COOR⁹ ;R¹¹ ishydrogen or C₁ -C₁₈ -alkyl, R¹² is C₁ -C₁₈ -alkyl, vinyl or isopropenyl,R¹³ is a C₈ -C₂₂ -alkyl, R¹⁴ is hydrogen or an organic radical such asis customarily formed in the free-radical polymerization of the startingmonomers, k is 0 or 1, x is from 1 to 12 and n is an even number m. 8.The monomer composition of claim 1, which further comprises one or morearomatic nitroso or nitro compounds.
 9. The monomer composition of claim1, which further comprises one or more costabilizers selected from thegroup consisting of phenothiazines, quinones, hydroquinones and theirethers, hydroxylamines, and phenylene-diamines.
 10. The monomercomposition of claim 6, wherein R⁸ is --CH₂ CH₂ --COO(CH₂)₁₁ --CH₃ or--CH₂ CH₂ --COO(CH₂)₁₂ --CH₃.
 11. The monomer composition of claim 6,wherein R⁹ is dodecyl or hexadecyl.
 12. The monomer composition of claim7, wherein R¹⁰ is C₁ -C₄ -alkyl.
 13. The monomer composition of claim 7,wherein R⁹ is hydrogen.
 14. The monomer composition of claim 7, whereinR¹² is vinyl, isopropenyl, or C₁₅ -C₁₇ -alkyl.
 15. A process forinhibiting the premature polymerization of monomers as claimed in claim1, which comprises adding at least one N-oxyl compound of a secondaryamine which does not bear hydrogen on the α-carbons to the monomers asclaimed in claim 14, in an amount effective as stabilizer.
 16. Theprocess of claim 15, wherein at least one tetraalkylpiperidine-N-oxylcompound of the formula (II) of claim 19 is added to the monomers. 17.The process of claim 15, wherein the vinyl-containing monomers to bestabilized are selected from the group consisting of N-vinylformide,N-vinylpyrrolidone, N-vinylcaprolactam, and C₁ -C₄ -alkyl vinyl ethers.